(1) Field of the Invention
This invention relates to a drive circuit of a brushless motor, and more particularly to a drive circuit of a brushless motor which is used as a fan motor of an air-conditioning unit for an automotive vehicle, for sampling a compartment temperature by sucking air at a predetermined location within a compartment of the vehicle.
(2) Description of the Related Art
Conventionally, among air-conditioning units for automotive vehicles, there is a type which samples air temperature at a predetermined location within a compartment of the vehicle and carries out temperature adjustment such that the temperature within the compartment becomes equal to a preset temperature. As a device for detecting the air temperature, an in-car sensor is known. The in-car sensor is comprised of a thermistor and an aspirator, and the aspirator is comprised of a fan for sucking air within the compartment and guiding the air to the thermistor, and a motor for driving the fan. The motor used for the aspirator is required to be quiet since a port of the aspirator for sucking air is arranged at a location near the head of an occupant, and therefore, a brushless motor is used therefor. An example of a circuit for driving such a brushless motor will be shown below.
FIG. 4 is a circuit diagram showing an example of a drive circuit of a conventional brushless motor.
This drive circuit of the brushless motor is a drive circuit for a motor including two drive coils L11, L12, and is comprised of two transistors TR1, TR2, resistances R11, R12, and capacitors C11, C12 forming an astable multivibrator circuit, and an oscillator circuit.
The drive coils L11, L12 are connected to collectors of the transistors TR1, TR2, and the collectors of the transistors TR1, TR2 and the capacitors C11, C12 are connected in cross connection within the oscillator circuit 1. The oscillator circuit 1 monitors a back electromotive force (braking torque) developed in the drive coils L11, L12 in response to the transistors R1, R2 and the capacitors C11, C12, and starts oscillation at a predetermined frequency when stoppage of the motor is detected, to supply its oscillating output to an external output terminal SG-OUT via a resistance R13.
When the power supply xc2x1V is applied to the transistors TR1, TR2, the transistors TR1, TR2 repeatedly carry out on/off operation at an oscillation frequency determined by a time constant of the resistances R11, R12 and the capacitors C11, C12, whereby electricity flows through the drive coils L11, L12 which are wound in respective opposite directions in an alternating manner. This makes it possible to continuously rotate the motor in a certain direction. During normal rotation, the oscillation is carried out at a shorter repetition period than a repetition period of a oscillation frequency determined by a time constant of the resistance R11 and the capacitor C11, for rotation of the motor.
When the back electromotive force of the drive coils L11, L12 of the motor is detected due to a locked state of the motor in which the rotation of the motor is stopped by some cause, the oscillator circuit 1 carries out self-oscillation at a predetermined frequency determined by the time constant of the resistance R11 and the capacitor C11, and delivers the oscillating output signal to an external circuit via the external output terminal SG-OUT thereof.
FIG. 5 is a circuit diagram showing another example of a drive circuit of a conventional brushless motor.
This drive circuit of the brushless motor is a drive circuit for a motor including a search coil L21 for detecting rotation thereof, and a drive coil L22 for driving the motor for rotation, and is comprised of transistors TR11 to TR13, resistances R21 to R23, a capacitor C21, and a diode D1.
The search coil L21 is connected between the bases of the transistors TR11, TR12 in current mirror circuit configuration, and detects a voltage induced by the motion of the magnet of the rotor. The transistor TR13 turns on when electric current is supplied via the resistances R22, R23 to its base to drive the drive coil L22, and turns off when the supply of electric current via the resistance R23 to its base is cut off by a turn-on operation of the transistor TR12 caused by the electromotive force detected by the search coil L21. Thus, the on/off operation of the drive coil L22 caused by the transistor TR13 generates a drive torque to thereby drive the motor for rotation.
However, although the type of the drive circuit of the conventional brushless motor having the unastable multivibrator circuit configuration is equipped with the oscillator circuit for notifying the stoppage state of the motor to the outside, it is complicated in circuit configuration and suffers from the problem that the oscillation frequency is not stable due to susceptibility to changes in temperature.
Further, although the type of the drive circuit of the brushless motor making use of the search coil is simple in circuit configuration and excellent temperature characteristics, it does not have the function of notifying the stoppage of the motor to the outside, and hence even when the motor is stopped during its operation of rotation, the stoppage of the rotation cannot be known from the outside.
The present invention has been made in view of the above points, and an object thereof is to provide a drive circuit for a brushless motor which is simple in construction and has a function of detecting the stoppage of rotation of the motor.
To accomplish the above objects, according to the present invention, there is provided a drive circuit of a brushless motor including a drive coil for driving a rotor portion of a magnet, and a search coil for detecting rotation of the rotor. The drive circuit of a brushless motor comprises a first operational amplifier that forms a Schmitt trigger circuit performing on/off operation based on a rotation detection signal generated by the search coil, and has an output thereof connected to one end of the drive coil and a second operational amplifier that forms an oscillator circuit performing inversion in synchronism with the on/off operation of said first operational amplifier, and oscillating at a predetermined frequency when said first operational amplifier does not perform the on/off operation, and has an output thereof connected to another end of the drive coil and at the same time to an output terminal to an external circuit.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.